I’ve been asked this question more times than I can remember. In most cases I have to answer that it’s very likely that your idea does already exist especially if it addresses a real need in the marketplace but that shouldn’t stop you from pursuing it.

I found the best answer to this question that I have seen in a very long time, it may be the best answer I’ve ever seen. It was written by Carlos Del Carpio as the answer on Quora. I haven’t change a thing.

“The founder of Dropbox was pitching in front of an investor one day, and the investor asked him: "there are similar companies out there doing the same as dropbox, why should I invest in just another similar company?"

His reply: "Yes. There are similar companies out there doing the same as dropbox. But do you use any of them?"

"No."

"Why?"

"Because they are bad."

"Ok. That's what Dropbox wants to solve."

So bottomline:Ask your potential customers.

Any business idea is a solution to someone's problem out there. Ask them if they are solving it, how they are solving it, and if they would be willing to pay to have it solved.”

It doesn’t matter how your business idea is gorgeous , if the market is not ready for it, or niche is full - you will not get any profit, at least not now. To check idea on the "strength" we must give it to the public.
The first way is to try out the idea “offline” . One of the most effective offline methods is pitching ideas.
The essence of pitching - to tell all the features of your idea during 2 - 5 minutes. The benefits of this method are: experts give you advices; the audience is involved in the discussion; you can find potential investors for your business; you will get community support and find like-minded people. For example, Pitch NYC (http://pitch-nyc.com/), Flash Pitch, (https://www.flashpitch.nyc/) The Biggest Startup Pitch event in the USA – 850 start up pitches for 150 Expert DOJO Member (https://expertdojo.com/events/... and good article from Forbes 7 Startup Competitions You Need To Know (http://www.forbes.com/sites/il...
The second way, of course, to try out the idea “online”. Probably the most effective method to identify opportunities online - to use crowdfunding platforms and B2B, B2C marketplaces for your business ideas.
1. Crowdfunding. The great thing about this process is that you are forced to go with your idea "to people" much earlier than usual. At the same time you are getting feedback and can correct your project much faster. It is very useful for early stage startups, because it can save entrepreneur’s time in the future and money that he could spend on the "Debug" project. The feedback in the process of crowdfunding can help to understand if the campaign will be successful, or unsuccessful. Examples: Kickstarter (https://www.kickstarter.com/), Crowdfunding for Small Businesses (https://www.fundable.com/), and recommendations from Forbes Top 10 Crowdfunding Sites For Fundraising (http://www.forbes.com/sites/ch...
2. B2B, B2C marketplaces.
-The presence of members in the form of "internet community" allows to share information among all market participants.
-Online reviews and ratings can be good feedback tools of risk management for companies.
-Easy selection of proposals, the speed of decision-making, the way of communication via electronic channels help simplify communication.
For example, one which can be usefull for small- and midsize companies - Opporty (https://opporty.com/). Its B2B and B2C approaches help gain a strong foothold on the local market, appealing to local customers in the specific niche. Or if you want to compete with large and stable companies you can try large platforms, such as Magento (https://magento.com/).
The combination of these methods will help you to create a complete picture of "readiness" the ideas for implement.